1. Field of the Invention
The present invention relates to electrets, and more particularly, to layered electrets retaining a high surface potential.
2. Claim for Right of Priority
I hereby claim my right to the filing date of my corresponding application filed in the United Kingdom on May 20, 1981 under the filing Ser. No. 8,115,402.
3. Description of the Prior Art
An electret is a known device comprising a body of dielectric material within which electric polarization is established to create a persistent electrostatic field. Such electrets can be employed in instances where charged dielectric or electrostatic fields are required without the employment of a dynamic generating system. In particular, charged surfaces are of use as an alternative to magnetic attachment, for example, serving the use of bulletin boards or other display functions. Furthermore, charged surfaces may be used in filtering systems to collect airborne dust and in many other applications.
In the past the knowledge that certain dielectric materials may be electrostatically polarized has been well developed. Furthermore, techniques for polarized dielectric materials are known and, for example, stable synthetic materials such as polymethylmethachrylate, polystyrene, polythene, polyethylene terephthalate or polymers of vinyl chloride or vinyl acetate or copolymers or combinations thereof, have had extensive success in forming stable electrets.
Typically, such materials were polarized through the following steps:
1. By raising the temperature of the material to an excitation temperature known as the "polarizing temperature." PA1 2. By applying an electric field to produce a polarizing charge. PA1 3. By allowing the material to cool in the presence of the electric field to thus fix the charge. PA1 1. Contact is made between the inner faces of the assembly, thereby initiating an electrical charge at the initial points of contact. PA1 2. As the pressure develops the faces are brought into closer contact with a further development of electrical charge. Simultaneously, the air between the faces of the inner sides of the element is being excluded and exuded at the outer edges of the assembly, permitting a progressively greater degree of contact between the faces. PA1 3. The exclusion of all air from between the faces is followed by the yielding, through heat, of the exterior sheets. Since the softening point of PET is higher than that of the copolymer exterior sheets, the copolymer sheets will be displaced under pressure to flow in a lateral plane so that the faces of the two exterior sheets will in effect migrate over the face of the still rigid PET interior sheet. PA1 4. This lateral movement creates friction which is known to have a maximum effect when the rubbing surfaces are in true or near true contact. PA1 5. The softened exterior surfaces, furthermore, also act to enclose any particulate debris that is always found in any commercial process which, under pressure, will bring about the deformation to allow the completion of the intimate contact between the faces. It has also been found that the rupturing of the polymer faces by such particulate matter will contribute further to the generation of charge. PA1 6. Simultaneously the temperature of the elements or of the sheets is being raised to the temperature at which the polymers are polarized in the presence of the tribolelectrical forces now developed. PA1 7. Concurrently the two outer edges of the exterior sheets are fused together so that a hermetic container is formed sealing off the charged interfaces from outside atmosphere. PA1 8. While still applying pressure the heat source may then be disconnected to begin a cooling cycle while the fused edges are still compressed. PA1 9. At the completion of the above process the assembly is removed from the press. The three sheets are still firmly in contact due to the following effects: PA1 (a) the electrostatic force developed between the interfaces in accordance with Coulombic law, due to the opposite polarities in contact; and PA1 (b) the negative atmosphere developed as result of the evacuation of air from between the interfaces.
In each instance the foregoing technique results in an electrically polarized structure, known as an "electret," which functionally and theoretically may be considered as the electrical equivalent of a permanent magnet.
These past developments in this art were primarily directed to improvements in the formulation in the materials and methods of manufacturing same with one common feature: most, if not all the prior art techniques utilize an externally impressed electrical field in order to produce the polarized charge. Thus the prior art is replete with structures wherein electrical charges are produced by clamping the dielectric material between conducting surfaces and impressing thereon a high potential source of direct current, as for example, that disclosed in U.S. Pat. Nos. 3,660,736 and 3,949,178, or by bombardment of the surface of the dielectric material with an electron beam or irradiation from a radioactive source of energy (see for example, U.S. Pat. No. 3,930,066). In addition the literature containing references to electrets can be found in the following: "The Theory of Electric Polarization," Volume 2, C. J. F. Boettcher; "Contact and Frictional Electrification," W. R. Harper, 1967, Oxford University Press; "Static Electrification" by Loeb, German, 1958; and "Electrostatics and Its Application," A. D. Moore, 1973, John Wiley, Inc., U.S.A.
While the foregoing prior art techniques are suitable for their intended purpose, they all suffer from the limitation inherent in charging thick films. In particular, it has been found that the charging voltage necessary to polarize thick films is of sufficient intensity to destroy the material itself. Thin films, on the other hand, suffer deterioration due to ohmic leakage and absorption of ions from the surface with the resulting reorientations of the dipole. Furthermore, in order to produce an electret having a long life one must seek a large overall volume resistivity. If the resistance is small the charge will disappear in a short time.
Thus techniques by which charge may be impressed on thick film, without the necessary high charging voltages, is sought in the art. It is such a technique that is described herein.